ENGLISH ABSTRACT: Single photon emission computed tomography (SPECT) with a superior contrast
resolution has been shown to be more sensitive and specific with a lower nondiagnostic
rate than planar imaging in many nuclear medicine studies but it is still not
being routinely implemented in V/Q studies at many centres including Tygerberg
Hospital. There are many studies on V/Q SPECT using Technegas as a ventilation
agent but very limited studies available on 81m Kr gas.
Aim: To clinically compare conventional planar and SPECT V/Q imaging using 81mKr
gas in the diagnosis of pulmonary embolism, with CTPA as the gold standard.
Patients and Methods: All patients referred with clinical suspicion of pulmonary
embolism were assessed. The inclusion criteria were normal chest radiograph,
normal renal function and no contrast allergy. Exclusion criteria were age below 18
years old, pregnancy, abnormal chest radiograph, abnormal serum creatinine/urea
levels and unstable patients. A Well’s score was assigned to each enrolled patient.
Perfusion scintigraphy was performed after intravenous injection 125 MBq of 99mTc
MAA. Ventilation scintigraphy was performed with 81mKr gas. On a dual head
camera, SPECT was done before planar acquisition, while perfusion was done
before ventilation imaging in the same position. Planar V/Q images consisted of 6
standard views. All V/Q SPECT images were reconstructed using ordered-subset
expectation-maximization (OSEM) algorithm and a post-reconstruction 3D
Butterworth filters were applied. V/Q Planar and V/Q SPECT images were later
evaluated and reviewed separately and reported based on recent EANM guidelines
blinded to the CTPA results.
All patients underwent multi-slice CTPA examinations on a 40-detector row scanner.
The images were later assessed and reported blinded to the V/Q results.
Statistical analysis was done using the Fisher exact test for comparison of
categorical variables and the one-way ANOVA for continuous variables (p<0.05 was significant). Results: A total of 104 consecutive patients were referred with clinical suspicion of
pulmonary embolism. Seventy-nine patients were excluded from this study mostly
due to abnormal serum creatinine/urea levels. Only 25 patients were included in this
study, with a mean age of 48 ± 19 years, and 64% being females. When compared
to CTPA as gold standard, the prevalence of PE was 16% [5% – 37% at 95% CI],
sensitivity 75% [21% – 99% at 95% CI], specificity 90% [68% – 98% at 95% CI],
positive predictive value 60% [17% – 93% at 95% CI], negative predictive value 95%
[73% – 100% at 95% CI] and diagnostic accuracy 88% [69% – 97%at 95% CI] for
both V/Q Planar and SPECT. V/Q Planar showed a lower reader confidence i.e.
could only clearly resolve 72% of cases compared to V/Q SPECT, which could
precisely interpret all cases, showed more and better delineated mismatch vs match
and segmental vs non-segmental defects. All patients who were scored as PE
unlikely on Wells’ score (4) had PE ruled out on CTPA (p=0.04581) as well as 89%
of patients on V/Q SPECT and V/Q Planar.
Conclusion: Based on this study, V/Q Planar and V/Q SPECT have a similar
diagnostic performance in patients with a normal or near normal chest X-rays.